158 



BIOLOGY AND TECHNIQUE 



other tissues are more suitable for this purpose than liver tissue since 

 the latter is not easily obtained in a sterile condition, bacteria 

 often getting into it during life through the portal circulation. The 

 action of the tissues depends probably upon its great reducing 

 power. 



THE INCUBATION OF CULTURES 



After bacteria have been transferred to suitable culture media, it 

 is necessary to expose them to a temperature favorable to their develop- 

 ment. In the case of many saprophytes, 

 the ordinary room temperature is suffi- 

 ciently near the optimum to obviate the 

 use of any special apparatus for maintain- 

 ing a suitable temperature; in the case 

 of most pathogenic bacteria, however, 

 the body temperature of man, 37.5° 

 C, is either a necessary requirement 

 for their growth, or at any rate 



I III favors speedy and characteristic develop- 



I ment. 



m For the purpose of obtaining a uniform 



H temperature of any required degree, the 



apparatus in general use is the so-called 

 incubator or thermostat. This may be 

 adjusted for gelatin cultivation at 20 to 

 22° C, or for agar, broth, or other media 

 at 37.5° C. 



Incubators, while varying in detail, 

 are all constructed upon the same prin- 

 ciples. They consist of double-walled copper chambers, which are 

 fitted with a set of double doors, the outer being made of asbestos- 

 covered metal, the inner of glass. (See Fig. 41.) The space be- 

 tween the two walls is filled with water, which, being a poor 

 heat conductor, tends to prevent rapid changes of temperature 

 within the chamber as the result of changes in the external 

 environment. Both walls are perforated above by openings to 

 admit thermometers into the interior and one wall is perforated 

 so that a thermo-regulator may be inserted into the water 

 jacket. The under surface of the chamber is heated by a gas 



Fig. 42. 



Fig. 43. 



Fig. 42. — Thekmo-requlator. 

 (Lautenschlager.) 



Fig. 43.- 



-Thermo-regtilatob. 

 (Reichert.) 



